1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to compensating for vortex shedding in a disk drive by modifying the thickness of an actuator arm.
2. Description of the Prior Art
Vortex shedding is a phenomenon manifesting in disk drives due to the air flow caused by the disk(s) spinning. A prior art head disk assembly 2 is shown in FIG. 1 as comprising a plurality of disks 4 and associated heads 6 actuated over the disks 4 by actuator arms 8 rotated about a pivot 10 by a voice coil 11 and magnets (not shown) of a voice coil motor. The air flowing around the actuator arms 8 generates an undesirable vertical oscillation which hinders the ability to maintain the heads 6 over the centerline of a track (i.e., tracking) during read and write operations. Specifically, when the air leaves the trailing edge of the actuator arms 8, it rolls up into coherent vortex structures aligned with the axis of the actuator arms 8. The vortex structures form alternately at the top and bottom of the trailing edges at a particular frequency referred to herein as the xe2x80x9cvortex sheddingxe2x80x9d frequency. The vortex shedding frequency (f) is related to the velocity (U) of the air flow around the actuator arms 8 and the thickness (D) of the actuator arms 8. The relationship between these three parameters has been well studied for simple flows, such as a circular cylinder aligned perpendicular to a uniform velocity flow. For such a flow, it was determined that fD/U is a constant for a wide range of f, D and U. This result can be found in a variety of textbooks on fluid mechanics.
By virtue of their rapid rotation, a low pressure region is also formed at the center of the vortex structures. The low pressure region, in conjunction with the alternate formation of the vortex structures at the vortex shedding frequency, results in an undesirable vertical oscillation of the actuator arms 8. When the frequency of the vertical oscillation aligns with a structural resonance of the head stack assembly, large amplitude motion results.
There is, therefore, a need to compensate for vortex shedding in a disk drive in order to attenuate the perturbing effect on the actuator arms, thereby improving tracking performance.
The present invention may be regarded as a method of designing an actuator arm of a head stack assembly for use in a disk drive comprising a disk, the actuator arm for positioning a head over the disk while the disk is rotating at a selected spin-rate. The actuator arm having a leading edge and a trailing edge and a selected thickness, and the head stack assembly having a structural resonance frequency. The disk is rotated such that the disk generates air flow around the actuator arm while the disk is spinning at the selected spin-rate. The actuator arm positions the head over the disk while the disk is spinning at the selected spin-rate, wherein the air flow travels from the leading edge to the trailing edge of the actuator arm such that the air flow causes vortex shedding at a vortex shedding frequency resulting in an undesirable force being applied to the actuator arm when the vortex shedding frequency is aligned with the structural resonance frequency. A perturbing effect due to the undesirable force applied to the actuator arm is measured. The thickness of the actuator arm is then modified and the thickness data stored. The process is then reiterated for a plurality of different actuator arm thickness values. A manufacturing thickness is then selected for the actuator arm from the stored thickness data to compensate for the undesirable force that is applied to the actuator arm due to the vortex shedding.
The present invention may also be regarded as a disk drive comprising a disk and a head disk assembly. The head disk assembly comprises an actuator arm for positioning a head over the disk while the disk is rotating at a selected spin-rate. The actuator arm having a leading edge and a trailing edge and a selected thickness, and the head stack assembly having a structural resonance frequency. The actuator arm is designed using an iterative design procedure. The disk is rotated such that the disk generates air flow around the actuator arm while the disk is spinning at the selected spin-rate. The actuator arm positions the head over the disk while the disk is spinning at the selected spin-rate, wherein the air flow travels from the leading edge to the trailing edge of the actuator arm such that the air flow causes vortex shedding at a vortex shedding frequency resulting in an undesirable force being applied to the actuator arm when the vortex shedding frequency is aligned with the structural resonance frequency. A perturbing effect due to the undesirable force applied to the actuator arm is measured. The thickness of the actuator arm is then modified and the thickness data stored. The processes is then reiterated for a plurality of different actuator arm thickness values. A manufacturing thickness is then selected for the actuator arm from the stored thickness data to compensate for the undesirable force that is applied to the actuator arm due to the vortex shedding.